The present invention generally relates to charged particle beam exposure methods and apparatuses, and more particularly to a charged particle beam exposure method which exposes a basic pattern on an object by a single shot of a charged particle beam using a mask which is provided with an opening of the basic pattern which forms a unit of a repeating pattern and to a charged particle beam exposure apparatus which uses such a method.
Recently, due to the further increase in the integration density of large scale integrated circuits (LSIs), a new exposure apparatus which uses a charged particle beam such as an electron beam has been proposed to replace the photolithography technique which was popularly used over the years to expose fine patterns. The charged particle beam exposure apparatus scans the object by deflecting a charged particle beam which has a rectangular cross section which is variable, and draws a desired pattern on the object. The charged particle beam exposure apparatus is provided with a pattern generation function for forming a pattern from a pattern data. However, because the charged particle beam exposure apparatus draws the pattern by connecting rectangular shots of the charged particle beam, there was a problem in that the number of exposure shots per unit area generally increases as the pattern size becomes smaller, thereby reducing the throughput.
On the other hand, a semiconductor device which requires extremely fine patterns such as a 64-Mbit dynamic random access memory (DRAM) has extremely fine patterns but a majority of the area to be exposed include a repetition of basic patterns. For this reason, if the basic pattern which forms a unit of the repeating pattern can be generated by a single shot regardless of the complexity thereof, it becomes possible to carry out the exposure with a constant throughput independently of the fineness of the patterns.
Accordingly, in order to overcome the above described problem and realize a satisfactory throughput even when exposing the extremely fine patterns, the so-called block exposure method has been proposed. The block exposure method irradiates a transmission mask which includes the basic pattern by the charged particle beam so that the basic pattern is exposed in a single shot, and the exposed basic patterns are connected to form the repeating pattern. The transmission mask is also referred to as a block mask or a stencil mask.
In the charged particle beam exposure apparatus employing the block exposure method, a plurality of kinds of basic patterns are normally formed in the transmission mask in order to efficiently carry out the exposure. However, it is desirable that the number of kinds of patterns which can be exposed by use of the transmission mask is greater than the number of kinds of basic patterns formed in the transmission mask.
FIG. 1 generally shows an example of a conventional charged particle beam exposure apparatus which employs the block exposure method. This example uses an electron beam as the charged particle beam. For example, this conventional electron beam exposure apparatus is proposed in Hans C. Pfeiffer, "Recent Advances in Electron-Beam Lithography for the High-Volume Production of VLSI Devices", IEEE Transactions on Electron Devices, Vol.ED-26, No. 4, April 1979.
In FIG. 1, an electron beam emitted from an electron gun 51 passes through a rectangular opening of a shaping plate 52 and the cross sectional shape of the electron beam is shaped into a rectangular shape. Further, the electron beam is converged by an electron lens 53 and is deflected by a deflector 54a which is used to select a pattern, so that the electron beam passes through a desired pattern part of a stencil mask 55.
On the other hand, a central processing unit (CPU) 62 reads out a pattern data which is stored in a memory unit 66 and inputs the pattern data to an exposure controller 64 via a bus 63. The exposure controller 64 supplies to an amplifier and converter (AMP/DAC) 65 a block deflection data which is dependent on a basic pattern which is to be exposed from the pattern data. The AMP/DAC 65 is made up of an amplifier (AMP) and a digital-to-analog converter (DAC). The AMP/DAC 65 amplifies and converts the block deflection data into an analog deflection control signal which is supplied to the deflector 54a. Hence, the electron beam is deflected by the deflector 54a so as to irradiate the selected basic pattern. In addition, the deflection control signal also drives a deflector 54b which is used to select a pattern.
As a result, the cross sectional shape of the electron beam is shaped into the pattern of the stencil mask 55 and is returned to the same optical axis as the electron beam emitted from the electron lens 53 by the converging function of an electron lens 56 and the deflection function of the deflector 54b. The cross section of the electron beam is thereafter reduced by a reduction lens 57.
The electron beam transmitted through the reduction lens 57 reaches an electron lens 59 via a circular opening of a plate 58. The circular opening limits the electron beam so as to determine the optical axis. The electron lens 59 converges the electron beam and a projection lens 60 exposes the electron beam at a predetermined position on an object surface 61. A deflector (not shown) provided at the projection lens 60 deflects the electron beam so as to scan the exposure position on the object surface 61.
FIGS. 2A and 2B respectively show a plan view and a cross sectional view of the stencil mask 55. The stencil mask 55 is made up of plate made of a semiconductor such as silicon (Si), a metal or the like. The thickness of the stencil mask 55 is approximately 20 .mu.m at a pattern forming part 71.
As shown in FIG. 2A, the pattern forming part 71 of the stencil mask 55 includes basic patterns 72, 73 and 74 which form units of the repeating patterns. The basic pattern 72 is made up of two openings 72a and 72b having the shape shown, and a blocking part 72c. The basic pattern 73 is made up of two openings 73a and 73b having the shape shown, and a blocking part 73c. In addition, the basic pattern 74 is made up of two openings 74a and 74b having the shape shown, and a blocking part 74c.
The stencil mask 55 further includes a rectangular opening 75 which is used to form a non-repeating pattern, as shown in FIG. 2A. This rectangular opening 75 is used to carry out the exposure by varying the irradiating position of the electron beam relative to the rectangular opening 75 to vary the cross sectional area of the shaped electron beam.
The basic patterns 72, 73 and 74 and the rectangular opening 75 are provided within a pattern forming region II which is smaller than a electron beam deflecting region I. The electron beam deflection region I is the region in which the electron beam can be deflected, and the pattern forming region II is the region in which a pattern can be exposed. At the time of the exposure, the electron beam is deflected to irradiate a selected pattern depending on the exposure pattern data, and the selected pattern is exposed on the object surface 61.
Actually, a plurality of pattern groups are provided within the electron beam deflecting region I of the stencil mask 55, where each pattern group is a collection of patterns such as the basic patterns 72 through 74 and the rectangular opening 75. For this reason, when selectively irradiating a single pattern within a different pattern group, the stencil mask 55 is moved so that this different pattern group falls within the electron beam deflecting region I.
Hence, although not shown in FIG. 1, the stencil mask 55 is placed on a movable stage and is moved in a vicinity of the optical axis of the electron optical system. In addition, the electron optical system from the electron gun 51 to the object surface 61 are provided within a vacuum column body, and in order to load the stencil mask 55 onto the stage without disturbing the vacuum state, a sub chamber (not shown) for use in mask loading is provided so that the column body and the sub chamber can be disengaged at a gate valve (not shown).
In the conventional electron beam exposure apparatus, one basic pattern made up of the openings 73a and 73b as shown in FIG. 3A is selected from the plurality of basic patterns of the stencil mask 55 and the electron beam irradiates an irradiating range 81 of the stencil mask 55 so as to expose this selected basic pattern. As a result, the basic pattern including patterns 82a and 82b shown in FIG. 3B is exposed on the object surface 61 by a single shot of the electron beam. It is thus possible to expose a repeating pattern which is made up of the basic patterns at a high speed by connecting such basic patterns.
Therefore, according to the conventional electron beam exposure apparatus, the variable rectangular pattern made by the rectangular opening 75 or any of the basic patterns provided in the stencil mask 55 may be generated by a single shot of the electron beam.
However, a part of the basic pattern may appear in a portion of the repeating pattern, particularly at an end portion of the repeating pattern. For this reason, it is desirable that parts 84a and 84b of the basic pattern can be exposed on the object surface 61 as shown in FIG. 4B by setting an irradiating range 83 at a position shifted relative to the stencil mask 55 which is provided with the openings 73a and 73b of the basic pattern as shown in FIG. 4A.
If the part of the basic pattern is frequently used, the pattern of this part itself may be provided as an independent basic pattern. However, if the part of the basic pattern is not used frequently, it is desirable not to provide this part itself as an independent basic pattern because the number of kinds of basic patterns to be provided in the stencil mask 55 would increase although the area of the stencil mask 55 is limited.
On the other hand, it is possible to expose only a part of the basic pattern by the variable rectangular pattern. But in this case, the number of shots would increase considerably and the exposure time would become extremely long.
Accordingly, when selectively carrying out the total exposure of the basic pattern as shown in FIG. 3A or the partial exposure of the basic pattern as shown in FIG. 4A in the conventional electron beam exposure apparatus, it is necessary to supply a first mask deflection data for selecting a certain basic pattern and a second mask deflection data for varying the irradiating range on the selected basic pattern to the same deflectors 54a and 54b. Basically, the first mask deflection data are discrete data. But while the first mask deflection data may vary in a large deflection range of 5 to 6 mm square by varying the least significant bit (LSB), the second mask deflection data needs to vary in a small deflection range of approximately 500 .mu.m square, for example. For this reason, if the first and second mask deflection data are added to form a single deflection data, an unnecessarily complex digital operation must be carried out in the AMP/DAC 65. In addition, it is difficult to determine the deflection efficiency of the first and second mask deflection data with the same accuracy in relation to the ratio with respect to the total amount of deflection, because the first mask deflection data basically varies in the large range in large steps while the second mask deflection data basically varies in the small range in small steps.